1. Field of the Invention
The invention relates to an overhead crane having a variable lifting capacity and redundant safety features, wherein the crane utilizes two independently reeved hoisting systems including a dual element hook to achieve its variable lifting capacity and redundant safety features. The invention disclosed is particularly useful in the environment of a nuclear power plant.
2. Description of the Prior Art
There are many overhead crane applications wherein the crane's safety factor, when supporting a load, is of utmost importance. One of the most common examples is the cranes utilized in steel mills. In that environment, heavy loads consisting of ladles filed with molten metal are supported by an overhead crane. Accordingly, the prior art cranes have painstakingly sought to prevent such loads from falling to the ground, should a portion or element of the overhead crane suffer a failure.
Examples of such prior art overhead cranes and their proposed solutions may be found in U.S. Pat. Nos. 1,148,323, issued to E. H. Kendall, on July 27, 1915; 1,181,155, issued to W. H. Morgan, on May 2, 1916; 2,370,834, issued to H. W. Ball, on Mar. 6, 1945; and 1,202,533, issued to D. Kendall, on Oct. 24, 1916; 2,271,427, issued to R. J. Harry, on Jan. 27, 1942; and 2,271,430, issued to R. J. Harry, on Jan. 27, 1942.
Each of the foregoing prior art patents utilize two hoist drums for supporting a load beneath the overhead crane. Additionally, these patents all utilize some form of gearing arrangement, whereby a failure associated with one hoist drum will not affect the operation of the other hoist drum. However, some of these prior art cranes; e.g., U.S. Pat. No. 1,181,155 and 1,148,323, utilize intermeshing gears disposed directly on the hoist drums. Accordingly, if a failure associated with one of the hoist drums were to occur, its failure would preclude the other hoist drum from rotating. Although the suspended load would not be dropped to the ground, the load would remain suspended in the air until such time as the broken hoist drum could be repaired. In the case of a steel mill, this could present a substantial problem, since the molten metal could solidify in the ladle, plus the attendant loss of time in having to stop utilizing that particular crane while it is being repaired.
The other patents previously referred to, while overcoming the problem associated with hoist drums having direct intermeshing gears, still do not provide a satisfactory solution to problems associated with either a hook or cable breaking. Although these patents provide a safety factor with respect to a single broken gear or hoist drum, there is no safety factor present if more than one gear or shaft should fail. Additionally, were the hooks suspended from these cranes to encounter any type of failure, the load suspended by that hook would fall to the ground.
In some overhead crane applications, a safety factor or safety feature which prevents the load being suspended from falling to the ground in the event of a single failure of some component of the crane may be sufficient. However, there are certain overhead crane applications wherein it is desirable and necessary to have safety factors, or safety features, which are redundant, whereby the load is prevented from falling to the ground if one or more of the elements of the overhead crane should fail.
One example of such an environment may be found in overhead cranes which are utilized in nuclear power plants, wherein such cranes are utilized for lifting and supporting radioactive nuclear materials above the floor of the nuclear power plant. Additionally, in such an environment, it is not desirable to merely prevent such a radioactive load from falling to the floor. Rather, it is extremely desirable to be able to either continue to lift or lower the radioactive load into its desired position, and not merely leave the load suspended while the crane is being repaired. Another factor to be taken into consideration in such an operating environment is that the overhead crane should likewise have a multiple safety factor, or safety features, associated with the lifting hook and its related cables, whereby if the hook or an associated cable should experience a failure, it will be possible to continue to raise or lower the radioactive load.
One proposed solution to such problems as are encountered in a nuclear power plant may be found in U.S. Pat. No. 3,786,935, issued to Joseph J. Vlazny et al., issued Jan. 22, 1974. Although this patent discloses an overhead crane which utilizes a "redundant or double load path crane hook" the crane hook is mounted upon a single load block. Thus, were a failure to occur affecting the load block structure, the intended purpose and safety features associated with the "redundant crane hook" would be useless and the suspended load would fall to the floor. Additionally, the suggested solution in this patent to achieve additional safety features is lacking in other respects. Since only one hoist drum is utilized, the crane will not function to raise or lower the radioactive load, if that hoist drum becomes disabled. Another problem could occur in the event the equalizer bar utilized by Vlazny encountered a failure. If that failure occurred, in all likelihood the suspended load would fall to the floor of the nuclear power plant. Whereas this patent provides a safety factor of one if a cable were to break; if both cables break, the load would naturally fall to the floor. Additionally, since all the sheaves in this disclosed crane do not rotate about parallel axes, the cables by which the load is suspended are subjected to unnecessary, additional torsional stresses which, it is believed, may affect the operating life of the cables.
Another factor to consider in utilizing an overhead crane in the environment of a nuclear power plant, or other large facility, is that such a crane is frequently utilized, not only in the operation of the ultimate plant, but the crane is also utilized to lift and lower other types of loads while the plant is being constructed. Normally, the loads encountered during the construction phase are of a larger magnitude than the loads encountered during plant operation. Accordingly, it is desirable to have an overhead crane which, in addition to having redundant safety features, also has a variable lifting capacity. U.S. Pat. No. 3,854,892, issued to Sigurd C. Mordre, on Dec. 17, 1974, is one example of an overhead crane which seeks to provide this variable lifting capacity. Basically, the crane disclosed in this patent utilizes a secondary trailer trolley. Although this crane does have a variable lifting capacity, there has been very little attempt made to provide this crane with adequate safety features, whereby this crane could be utilized in a nuclear power plant. For example, this crane could not withstand any failure occurring to its hook without the attendant result of the load falling to the floor of the plant. Furthermore, since only a single cable is utilized, failure of that cable would result in the suspended load falling to the ground.
Accordingly, prior to the development of the present invention, there has been no overhead crane available which provides an increased number of redundant safety features whereby an overhead crane could experience a failure occurring to more than one element of the overhead crane, and still continue to raise and lower a load suspended from the crane. Therefore, the art has sought an extremely safe overhead crane, suitable for use in the environment of a nuclear power plant, which crane possesses many redundant safety features.